Sheet feeding device and image forming apparatus

ABSTRACT

A sheet feeding device includes a sheet accommodating portion for accommodating a sheet, a feeding roller for feeding the sheet by rotation thereof in a state in which the feeding roller contacts an uppermost sheet accommodated in the sheet accommodating portion, a detecting portion for detecting a surface property of the sheet accommodated in the sheet accommodating portion, a changing means for changing contact pressure of the feeding roller to the uppermost sheet, and a controller for controlling the changing means so as to change the contact pressure on the basis of a result of the surface property of the sheet detected by the detecting portion.

FIELD OF THE INVENTION AND RELATED ART

The present invention relates to a sheet feeding device for feeding a sheet and an image forming apparatus including the sheet feeding device.

Conventionally, the image forming apparatus such as a printer, a facsimile, and a copying machine transfers an image formed, by an image forming portion, on the sheet fed by the sheet feeding device. The sheet feeding device feeds the sheet by rotating the feeding roller in a state in which the feeding roller is contacted to the sheet accommodated in a sheet accommodating portion. However, depending on a material, a thickness, and a size of the sheet, a feeding resistance between the feeding roller and the sheet becomes large. In this case, so-called non-feeding such that the sheet is not fed occurs in some instances.

In order to solve this problem, Japanese Laid-Open Patent Application 2014-181086 discloses a constitution in which a detecting means for detecting surface roughness of the sheet is provided and in which a speed of a feeding roller is changed on the basis of a detection result. Specifically, a feeding speed of the sheet discriminated that the surface roughness is large is made lower than a feeding speed of the sheet discriminated that the surface roughness is small.

However, in the above-described constitution, for example, the sheet feeding speed is increased when a sheet with small surface roughness such as coated paper prepared by subjecting a surface of the sheet to coating is fed, but in the case where contact pressure between the feeding roller and the sheet is small, there is a liability that a slip occurs between the feeding roller and the sheet. As a result, there is a liability that the sheet cannot be fed by the feeding roller and thus improper feeding occurs.

SUMMARY OF THE INVENTION

A principal object of the present invention is to provide a sheet feeding device capable of stably executing sheet feeding irrespective of a kind of a sheet.

According to an aspect of the present invention, there is provided a sheet feeding device comprising: a sheet accommodating portion configured to accommodate a sheet; a feeding roller configured to feed the sheet by rotation thereof in a state in which the feeding roller contacts an uppermost sheet accommodated in the sheet accommodating portion; detecting means configured to detect a surface property of the sheet accommodated in the sheet accommodating portion; changing means configured to change contact pressure of the feeding roller to the uppermost sheet; and a controller configured to control the changing means so as to change the contact pressure on the basis of a result of the surface property of the sheet detected by the detecting means.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic structural view of a printer of an embodiment 1.

FIG. 2 is a schematic sectional view of a sheet feeding portion in the embodiment 1.

Part (a) of FIG. 3 is a sectional view of an adjusting unit in the embodiment 1, and parts (b) and (c) of FIG. 3 are schematic views each for illustrating an operation mode of the adjusting unit.

FIG. 4 is a schematic structural view of a detecting device in the embodiment 1.

FIG. 5 is a control block diagram of the printer of the embodiment 1.

FIG. 6 is a flowchart showing a flow of an operation for adjusting contact pressure of a pick-up roller in the embodiment 1.

FIG. 7 is a graph showing a relationship between a feeding resistance between the pick-up roller and a sheet and feeding efficiency in the embodiment 1.

FIG. 8 is a graph for illustrating a deriving method of the feeding efficiency of the sheet in the embodiment 1.

FIG. 9 is a graph showing a relationship between a rotational speed, the contact pressure, and a feeding state of the pick-up roller in the embodiment 1.

FIG. 10 is a flowchart showing a flow of an operation for adjusting pressure of a roller pressure variable motor in the embodiment 1.

FIG. 11 is a flowchart showing a flow of an operation for adjusting a rotational speed of a pick-up roller in an embodiment 2.

Parts (a) and (b) of FIG. 12 are graphs each showing a relationship between the rotational speed, contact pressure, and a feeding state of the pick-up roller in the embodiment 2.

DESCRIPTION OF THE EMBODIMENTS

In the following, embodiments for carrying out the present invention will be described with reference to the drawings.

FIG. 1 is a schematic structural view of a printer 201 as an image forming apparatus in an embodiment 1 of the present invention. The printer 201 is constituted by including an apparatus main assembly 201A, an image forming portion 201B provided inside the apparatus main assembly 201A, an image reading device 202 provided horizontally above the apparatus main assembly 201A, and a controller 301 capable of controlling an entire operation of the printer 201. Further, between the image reading device 202 and the apparatus main assembly 201A, a discharge space S into which the sheet is discharged is formed. Further, in the apparatus main assembly 1A, a sheet cassette 2 as a sheet accommodating portion for accommodating a sheet P is provided. At a lower portion of the apparatus main assembly, a sheet feeding portion 230 for feeding the sheet P from the sheet cassette 2 as a sheet supporting portion by which the sheet P is supported is provided. In this embodiment, the controller 301 and the sheet feeding portion 230 function as a sheet feeding device in cooperation with each other.

The image forming portion 201B as an image forming means in this embodiment is of a four-drum full-color electrophotographic type and includes a laser scanner 210 and four process cartridges 211Y, 211M, 211C, and 211K. The process cartridge 211Y forms a toner image of yellow (Y). The process cartridge 211M forms a toner image of magenta (M). The process cartridge 211C forms a toner image of cyan (C). The process cartridge 211K forms a toner image of black (K). The process cartridges 211Y, 211M, 211C, and 211K have a common constitution except that colors of toners are different from each other. Therefore, the constitution of the process cartridge 211Y is described, and description of other process cartridges 211M, 211C, and 211K will be omitted. The process cartridge 211Y includes a photosensitive drum 212, a charging device 213, and a developing device 214 to which the toner is supplied from a toner cartridge 215. Further, the image forming portion 201B includes an intermediary transfer unit 201C provided on the process cartridge 211 and a fixing portion 220. The intermediary transfer unit 201C is constituted by including an intermediary transfer belt 216 wound about a driving roller 216A and a tension roller 216B. Incidentally, inside the intermediary transfer belt 216, a primary transfer roller 219 contacting the intermediary transfer belt 216 at a position opposing the photosensitive drum 212 is provided. The intermediary transfer belt 216 is rotated in an arrow A direction in FIG. 1 by the driving roller 216A driven by a driving portion. At a position of the intermediary transfer unit 201C opposing the driving roller 216A, a secondary transfer roller 217 for transferring the toner image, formed on the intermediary transfer belt 216, onto the sheet P is provided. Further, at a position above the secondary transfer roller 217, a fixing portion 220 is provided, and at portions leftward above the fixing portion 220, a first discharging roller pair 225A, a second discharging roller pair 225B, and a double side (printing) reversing portion 201D are provided. The double-side reversing portion 201D is provided with a reversing roller pair 222 capable of being rotated normally and reversely and a re-feeding passage R for feeding again the sheet P, on which the image is formed on one surface, to the image forming portion 201B.

Next, an image forming operation in the printer 201 will be described. First, an original read by the image reading device 202 or image information inputted to the printer 201 is converted into an electric signal and then is transmitted to the laser scanner 210 of the image forming portion 201B. The laser scanner 210 successively exposes surfaces of the photosensitive drums 212, electrically charged to a predetermined polarity and a predetermined potential uniformly by the charging devices 213, with laser light. By this, an electrostatic latent image based on the image information is formed on each of the photosensitive drums 212. The electrostatic latent image formed on each photosensitive drum 212 is developed by the associated developing device 214 and thus is visualized as a toner image. Then, the resultant toner images on the surfaces of the photosensitive drums 212 are successively transferred superposedly onto the intermediary transfer belt 216 by a primary transfer bias applied to the primary transfer rollers 219. By this, the toner images are formed on the intermediary transfer belt 216. Further, in parallel to the image forming operation, the sheet feeding portion 230 feeds the sheet P accommodated in the sheet cassette 2. The sheet P is fed from the sheet cassette 2 to a registration roller pair 240, and oblique movement of the sheet P is corrected by the registration roller pair 240. After the oblique movement of the sheet P is corrected, the sheet P is fed to the secondary transfer portion by the registration roller pair 240, and the toner images on the intermediary transfer belt 216 are transferred onto the sheet P by a secondary transfer bias applied to the secondary transfer roller 217. The sheet P on which the toner images are transferred is fed to the fixing portion 220 and is subjected to application of heat and rotational speed, so that an image is fixed on the sheet P. The sheet P on which the image is fixed is discharged to a discharge space S by the first discharging roller pair 225A and the second discharging roller pair 225B which are provided downstream of the fixing portion 220 with respect to the sheet feeding direction. The sheet discharged to the discharge space S is stacked on a stacking portion 223 provided at a bottom of the discharge space S. Incidentally, when images are formed on double surfaces (sides) of the sheet P, the image is fixed by the fixing portion 220 and thereafter is fed to the re-feeding passage R by the reversing roller pair 222. The sheet P fed to the re-feeding passage R is fed again to the image forming portion 201B, so that the image is formed on the back surface similarly as in the case of the front surface.

Next, with reference to FIG. 2, a constitution of the sheet feeding portion 230 as a feeding separation means for separating and feeding the sheets one by one in this embodiment will be described. FIG. 2 is a schematic sectional view of the sheet feeding portion 230. The sheet feeding portion 230 is comprised of the sheet cassette 2, a feeding unit 5, a feed roller 7, and a retard belt 8. The sheet cassette 2 is provided with a tray 3 which is rotatable about a rotation center 3 a and on which sheets P are stacked and a detecting device S1.

Further, at a lower portion of the tray 3, a raising and lowering plate 4 is provided and is raised and lowered by drive of a lifter motor M2 (see FIG. 5), so that the tray 3 is lifted up. The sheets P stacked on the tray 3 are contacted to a pick-up roller 6 by the lift-up of the tray 3 and thus are in a feedable state.

The feeding unit 5 includes the pick-up roller 6, the feed roller 7, and the retard roller 8 which are feeding rollers for feeding the sheets P stacked on the tray 3. The pick-up roller 6, the feed roller 7, and the retard roller 8 are driven by a feeding driving motor M3 (see FIG. 5) which is a common driving source. The pick-up roller 6 is mounted on a pick-up roller shaft 6A extending from a raising and lowering plate 11, and the raising and lowering plate 11 is rotatable about a feeding driving shaft 12. Further, the raising and lowering plate 11 is urged from above by a pick-up spring 10 which is an urging means, and by this urging force, the pick-up roller 6 urges the sheets P. A bearing surface of the pick-up spring 10 is fixed to an adjusting unit 14 described later, and an operation (working) large of the pick-up spring 10 can be changed by the adjusting unit 14. When the sheets P are fed, the feeding driving motor M3 is driven in a state in which the pick-up roller 6 is contacted to an uppermost sheet P, of the surfaces P stacked on the tray 3, by a predetermined urging force, so that the pick-up roller 6 is rotated. The uppermost sheet P feed by contact and rotation of the pick-up roller 6 is fed to a separation nip 13 formed by press-contact between the feed roller 7 and the retard roller 8. The sheets P are separated and fed one by one by the feed roller 7 and the retard roller 8 through the action of a torque limiter. A separating means in this embodiment is constituted by including the separation nip 13 formed by the feed roller 7 and the retard roller 8. Incidentally, this embodiment is also applicable to a constitution in which the separation nip 13 is formed by a separation pad attached to the sheet cassette 2 and a roller such as the pick-up roller 6 provided so as to be contactable to the separation pad, in place of the feed roller 7 and the retard roller 8.

Next, a constitution of the adjusting unit 14 for adjusting a force (contact pressure of the pick-up roller 6) for urging the sheet P in a state in which the pick-up roller 6 contacts the sheet P will be described. Part (a) of FIG. 4 is a sectional view of the adjusting unit 14 as an adjusting means in this embodiment. Part (b) of FIG. 3 is a sectional view for illustrating an operation mode of the adjusting unit 14 when the contact pressure of the pick-up roller 6 is made small. Part (c) of FIG. 3 is a sectional view for illustrating the operation mode of the adjusting unit 14 when the contact pressure of the pick-up roller 6 is made large. As shown in part (a) of FIG. 3, the adjusting unit 14 includes a rotation shaft 15, a slider 18, a restricting guide 16, a slider sensor 19, and a roller pressure variable motor M1 (see FIG. 5). The slider 18 is provided with a helical groove engaging with the rotation shaft 15 and a rotation stopper 17. The rotation shaft 15 is rotatable along the groove of the slider 18. The restricting guide 16 restricts rotation of the slider 18 while permitting translation (parallel displacement) of the slider 18 by engagement with the rotation stopper 17. For that reason, when the rotation shaft 15 is rotated, the slider 18 is transferred in an arrow A direction of part (a) of FIG. 3 along the restricting guide 16. On a bottom of the slider 18, a bearing surface of the pick-up spring 10 is provided, and when the slider 18 is translated, an operation length of the pick-up spring 10 changes. The rotation shaft 15 is driven by the roller pressure variable motor M1 (see FIG. 5). The roller pressure variable motor M1 is rotated by a predetermined number of rotations, and the slider 18 is translated by the rotation of the rotation shaft 15, whereby the operation length of the pick-up spring 10 can be changed to an arbitrary length. A spring in this embodiment is the pick-up spring 10, and a moving member in this embodiment for expanding and contracting the pick-up spring 10 is constituted by the rotation shaft 15, the restricting guide 16, the rotation stopper 17, and the slider 18.

A position of the slider 18 is determined on the basis of the number of rotations of the roller pressure variable motor M1 from a reference position of the adjusting unit 14. The restricting guide 16 is provided with the slider sensor 19 for detecting the presence or absence of the slider 18 with respect to a direction perpendicular to an axial direction of the rotation shaft 15. In the case where the slider sensor 19 is turned on, the rotation shaft 15 is rotated clockwise. When the rotation shaft 15 is continuously rotated clockwise, an upper surface of the slider 18 reaches an upper surface of the restricting guide 16, so that the slider sensor 19 is switched from an ON state to an OFF state. In part (a) of FIG. 3, a position when the upper surface of the slider 18 and the upper surface of the restricting guide 16 are flush with each other is set at the reference position of the adjusting unit 14. Further, when the rotation shaft 15 is rotated clockwise from the position of part (a) of FIG. 3, the slider 18 is translated in a direction in which the operation length of the pick-up spring 10 is made long. By this, an urging force of the pick-up spring 10 against the pick-up roller 6 becomes small, so that the contact pressure of the pick-up roller 6 to the sheet P becomes small (part (b) of FIG. 3). On the other hand, when the rotation shaft 15 is rotated counterclockwise from the position of part (a) of FIG. 3, the slider 18 is translated in a direction in which the operation length of the pick-up spring 10 is made short. By this, an urging force of the pick-up spring 10 against the pick-up roller 6 becomes large, so that the contact pressure of the pick-up roller 6 to the sheet P becomes large (part (c) of FIG. 3). In the adjusting unit 14, by such an operation, the contact pressure of the pick-up roller 6 to the sheet P can be adjusted to an arbitrary value.

Next, with reference to FIG. 4, a constitution of the detecting device S1 which is a detecting means for detecting a surface property of the sheet P will be described. FIG. 4 is a schematic sectional view of the detecting device S1. The detecting device S1 is a device (apparatus) capable of measuring a three-dimensional shape of the sheet P by utilizing the laser light and includes an optical unit 23, an objective lens 21, a lens inserting port 20, and a sheet holder 22. The detecting device S1 is disposed inside the sheet cassette 2. The optical unit 23 is constituted by including the confocal laser optical element 231 for subjecting the sheet P to laser scanning and the light receiving element 232 for receiving reflected light from the surface of the sheet P. The light receiving element 232 detects a change in light quantity of the reflected light from the surface of the sheet P and outputs a signal. The controller 301 described later (see FIG. 5) records a position, as height information, where brightness of the reflected light detected on the basis of the signal outputted by the light receiving element 232 becomes maximum. A surface of an object placed on the sheet holder 22 is two-dimensionally scanned with the laser light of the confocal laser optical element 231. Then, the controller 301 synthesizes pieces of height information at respective measuring positions in a scanning surface of the confocal laser optical element 231, and thus acquires a surface profile of the object placed on the sheet holder 22. Here, the surface profile refers to information indicating a surface property (characteristic) of the object placed on the sheet holder 22, and for example, as regards the sheet P, the sheet profile includes information indicating characteristics of the sheet P, such as a three-dimensional shape, surface roughness, scars, and creases. The sheet P is held in a state in which the sheet P is pressed against the bearing surface of the sheet holder 22, and the three-dimensional shape of the sheet P is such that the sheet P is not moved during measurement thereof.

Incidentally, as the detecting device S1, a constitution in which the sheet holder 22 is moved in a height direction during the measurement of the three-dimensional shape of the sheet P and in which a contour curve of the sheet P is detected on the basis of height information of a stage and a brightness value may also be employed. Further, as a constitution other than this constitution, a constitution in which a sensor for detecting a position of the sheet P with respect to the height direction is provided in the detecting device S1 and in which the contour curve of the sheet P is detected on the basis of the height information and the brightness value may also be employed. In the case where the contour curve of the sheet P is used, a constitution in which in the detecting device S1, only the surface roughness of the sheet P is detected on the basis of the contour curve of the sheet P may also be employed. The contour curve of the sheet P includes a first parameter indicating a difference between a highest point and lowest point with respect to the height direction in the contour curve of the surface of the sheet P and a second parameter indicating an interval of unevenness in the contour curve of the surface of the sheet P. Incidentally, in addition thereto, for example, an optical element such as a CIS is provided in place of the confocal laser optical element 231 and the light receiving element 232, and a surface image of the sheet P may also be picked up. In this case, the picked-up surface image of the sheet P is analyzed, and a portion where a change brightness value between adjacent pixels exceeds a threshold is extracted as a characteristic point, so that the three-dimensional shape and the surface roughness of the sheet P and the presence or absence of the scars, the creases, and the like of the sheet P may also be discriminated.

Next, with reference to FIG. 5, a constitution in which feeding of the sheet P from the sheet feeding portion 230 in the printer 201 will be described. FIG. 5 is a control block diagram of the printer 201. The controller 301 includes a CPU 303 as a calculating (computing means) and a memory 302 for holding (storing) a control program or the like and a setting value and also functioning as a working area of the CPU 303. The controller 301 is connected to the roller pressure variable motor M1, the lifter motor M2, and the feeding driving motor M3 via a driver 306. The controller 301 controls a display of an operating portion 304 and drive of each of the roller pressure variable motor M1, the lifter motor M2, and the feeding driving motor M3. A detection result of the detecting device S1 is inputted to the controller 301. As the roller pressure variable motor M1, a stepping motor may also be used.

Next, a flow of an operation for adjusting the contact pressure of the pick-up roller 6 to the sheet in the sheet feeding portion 230 will be described. FIG. 6 is a flowchart showing the flow of the operation for adjusting the contact pressure of the pick-up roller 6. The flowchart of FIG. 6 is principally executed by the controller 301. When a job for forming the image on the sheet is inputted (S1), first, the detecting device S1 detects a surface property of the sheet (S2). Then, the CPU 303 calculates a relation curve indicating a relationship between a feeding resistance and feeding efficiency between the sheet and the pick-up roller 6 (S3). In this embodiment, the feeding resistance refers to a force acting so that feeding of the sheet is prevented when the sheet is fed, and includes a friction resistance between sheets and a friction resistance between the sheet and the pick-up roller 6, and the like. Further, the feeding efficiency indicates a ratio of a sheet feeding speed to a rotational speed of the pick-up roller 6. That is, the feeding efficiency corresponds to a feeding distance of the sheet per unit rotation amount of the pick-up roller 6.

FIG. 7 is the relation curve showing the relationship between the feeding resistance and the feeding efficiency between the pick-up roller 6 and the sheet, and as represented by the following (formula 1), the relation curve can be expressed by a tangent function.

Vp/VR=1−Atan(B□P/F)   (formula 1)

Here, Vp is a sheet feeding speed, VR is a rotational speed of the pick-up roller 6, Vp/VR is feeding efficiency, and F is a feeding resistance between the pick-up roller 6 and the sheet. Further, P is contact pressure of the pick-up roller 6 to the sheet at the time of a start of the flow of FIG. 6. In the following description, the contact pressure of the pick-up roller 6 to the sheet at the time of the start of the flow of FIG. 6 is referred to as “start time pressure of the pick-up roller 6”. Further, each of a coefficient A and a coefficient B is a function having variables including the start time pressure of the pick-up roller 6, the rotational speed of the pick-up roller 6, and the surface property of the sheet. That is, the relative curve showing the relationship between the feeding resistance and the feeding efficiency between the sheet and the pick-up roller 6 in S3 in a curve represented on the basis of the surface property of the sheet detected in S2, the start time pressure of the pick-up roller 6, and the rotational speed of the pick-up roller 6. Further, as regards start pressure of the pick-up roller 6 and a based on rotational speed of the feeding roller, these values are determined in advance on the basis of information (basis weight and size) of the sheet fed. Incidentally, the coefficient A and the coefficient B are calculated by using the first parameter indicating the difference between the highest point and the lowest point with respect to the height direction in the contour curve of the surface of the sheet and the second parameter indicating the interval of the unevenness with respect to the horizontal direction in the contour curve of the surface of the sheet. Each of the first parameter and the second parameter is a value, of features of the surface property of the sheet, relating to the surface roughness of the sheet and is a value having interaction with the rotational speed of the pick-up roller 6 and the start time pressure of the pick-up roller 6. That is, depending on the values of the first parameter and the second parameter which relate to the surface roughness of the sheet, a shape of the contour curve showing the relationship between the feeding resistance and the feeding efficiency when the rotational speed of the pick-up roller 6 and the contact pressure of the pick-up roller 6 are changed.

(A) of FIG. 7 shows a relationship between the feeding resistance and the feeding efficiency when the contact pressure of the pick-up roller 6 is P1 and the rotational speed of the pick-up roller 6 is V1. From the relation curve shown at (A) of FIG. 7, when the contact pressure of the pick-up roller 6 is changed to P2 larger than P1, the relationship between the feeding resistance and the feeding efficiency changes as in a curve shown at (B) of FIG. 7. Further, from the relation curve shown at (A) of FIG. 7, when the rotational speed of the pick-up roller 6 is changed to V2 larger than V1, the relationship between the feeding resistance and the feeding efficiency changes as in a curve shown at (C) of FIG. 7.

On the other hand, the feeding resistance of the sheet is largely influenced by the frictional resistance between the sheets has high correlation to the surface property of the sheet, and therefore, the feeding resistance received by the sheet can be calculated on the basis of the surface property of the sheet detected in S2 of FIG. 6, the start time pressure of the pick-up roller 6 to the sheet, and the (formula 1) (S4).

Here, with reference to FIG. 7, a deriving (calculating) method of the feeding efficiency of the sheet by the pick-up roller 6 will be described. First, as described in S3 of FIG. 6, the relation curve showing the relationship between the feeding resistance and the feeding efficiency between the pick-up roller 6 and the sheet is calculated. (A) of FIG. 8 shows a relation curve between the feeding resistance and the feeding efficiency between the pick-up roller 6 and the sheet shown at (A) of FIG. 7. Then, as described in S4 of FIG. 6, the feeding resistance of the sheet is calculated on the basis of the surface property of the sheet and the start time pressure of the pick-up roller 6. (B) of FIG. 8 shows a value of the feeding resistance of the sheet calculated on the basis of the surface property of the sheet and the start time pressure of the pick-up roller 6. From a point of intersection between the relation curve of (A) of FIG. 8 and a rectilinear line of the feeding resistance of (B) of FIG. 8, the feeding efficiency between the pick-up roller 6 and the sheet can be acquired (FIG. 6, S5). (C) of FIG. 8 shows a rectilinear line of the feeding efficiency between the pick-up roller 6 and the sheet acquired in S5 of FIG. 6.

FIG. 9 is a graph showing a relationship between the contact pressure of the pick-up roller 6 to the sheet and a feeding state. In FIG. 9, a “stable feeding region represents a region in which the feeding is a predetermined value or more, and an “unstable region” represents a region in which the feeding efficiency is less than the predetermined value. When the rotational speed of the pick-up roller 6 is V1 and the start time pressure of the pick-up roller 6 to the sheet is P1, the feeding state of the sheet by the pick-up roller 6 is in the unstable region ((A) of FIG. 9). In order to feed the sheet stably in a state in which the rotational speed of the pick-up roller 6 is maintained at V1, it is desirable that the contact pressure of the pick-up roller 6 to the sheet is set at minimum pressure to the extent that non-feeding of the sheet from the sheet cassette 2 does not occur. In FIG. 9, predetermined at a boundary (solid line of FIG. 9) defining the stable feeding region and the unstable region corresponds to a threshold pressure which is a distance (predetermined distance) of an extent such that non-feeding of the sheet from the sheet cassette 2 does not occur. Accordingly, the contact pressure of the pick-up roller 6 to the sheet is set at a value (target pressure Pc) obtained by adding pressure of a margin with a variation in sheet to the pressure at the boundary defining the stable feeding region and the unstable region ((B) of FIG. 9). In this embodiment, thus, from a relation diagram showing the feeding state between the rotational speed of the pick-up roller 6 and the contact pressure of the pick-up roller 6 to the sheet, the target pressure Pc of the contact pressure of the pick-up roller 6 to the sheet is set (FIG. 6, S6). Then, the contact pressure of the pick-up roller 6 is changed by controlling the roller pressure variable motor M1 so that the contact pressure of the pick-up roller 6 to the sheet set in S6 becomes the target pressure Pc (S7), and then a sheet feeding job is started (S8).

Here, the control of the roller pressure variable motor M1 in S7 of FIG. 6 will be described using a flowchart of FIG. 10. FIG. 10 is the flowchart showing a flow of an operation for adjusting the pressure of the roller pressure variable motor M1. The flowchart of FIG. 10 is principally executed by the controller 301. First, the control of the roller pressure variable motor M1 is started, and the roller pressure variable motor M1 is rotated correspondingly to one pulse so that the rotation shaft 15 is rotated counterclockwise (S10). When the rotation shaft 15 is rotated counterclockwise, the slider 18 is translated downward. Then, the roller pressure variable motor M1 is continuously rotated until the detection signal of the slider sensor 19 becomes OFF (S11). When the upper surface of the slider 18 reaches the slider sensor 19, the detection signal of the slider sensor 19 changes from ON to OFF. In this embodiment, the upper surface of the slider 18 and the upper surface of the restricting guide 16 are flush with each other, and a position where the detection signal of the slider sensor 19 is switched from ON to OFF is a reference position.

Then, a pulse count of the roller pressure variable motor M1 is reset (S12), and the roller pressure variable motor M1 is reversely rotated until the detection signal of the slider sensor 19 switches from OFF to ON (S13). Then, on the basis of the contact pressure of the pick-up roller 6 set in S6 of FIG. 6, a target pulse number of the roller pressure variable motor M1 is calculated (S14). Here, the target pulse number of the roller pressure variable motor M1 and a limit pulse number of the roller pressure variable motor M1 are compared with each other (S15), and in the case where the target pulse number is larger than a value set as the limit pulse number (S15/N), the limit pulse number is used as the target pulse number (S16). In the case where the target pulse number is not more than the value set as the limit pulse number (S15/Y), the target pulse number calculated in S14 is used (S16). Then, the roller pressure variable motor M1 is rotated clockwise correspondingly to one pulse (S17), and then the pulse count of the roller pressure variable motor M1 is incremented by 1 (S18). At this time, whether or not the detection signal of the slider sensor 19 switches from ON to OFF (S19). In the case where the detection signal of the slider sensor 19 switches from ON to OFF, the roller pressure variable motor M1 is reversely rotated until the detection signal of the slider sensor 19 becomes ON (S21).

Thereafter, the pulse count and the target pulse number of the roller pressure variable motor M1 are compared with each other (S20). Then, when the pulse count of the roller pressure variable motor M1 is less than the target pulse number (S20/N), the sequence returns to S17, and the rotation of the roller pressure variable motor M1 is continued. On the other hand, when the pulse count of the roller pressure variable motor M1 is the target pulse number or more (S10/Y), the control of the roller pressure variable motor M1 is ended. In this embodiment, thus, the contact pressure of the pick-up roller 6 to the sheet is adjustable depending on the surface property of the sheet fed from the sheet feeding portion 230. At this time, it is desirable that the target pressure Pc of the contact pressure of the pick-up roller 6 is adjusted in a range in which a feeding force of the sheet by the pick-up roller 6 is smaller than a separating force of the sheet in the separation nip 13. By doing so, occurrence of the non-feeding of the sheet from the sheet cassette 2 and double feeding and a jam of the sheet(s) in the separation nip 13 can be suppressed. Therefore, in this embodiment, irrespective of a kind of the sheet, it becomes possible to provide a sheet feeding device capable of executing the feeding of the sheet stably.

Further, in this embodiment, the contact pressure of the pick-up roller 6 is adjusted so as to provide predetermined feeding efficiency on the basis of the surface property of the sheet, the rotational speed of the pick-up roller 6, and the contact pressure of the pick-up roller 6 to the sheet. As another example, the contact pressure of the pick-up roller 6 may also be adjusted by calculating the first parameter and the second parameter from the surface property of the sheet detected by the detecting device S1.

As described above, the first parameter indicates the difference between the highest point and the lowest point with respect to the height direction in the contour curve of the surface of the sheet. That is, the surface roughness of the sheet becomes large (rough) with a large first parameter. That is, when the first parameter is a first value, the surface roughness of the sheet is first roughness, and when the first parameter is a second value smaller than the first value, the surface roughness of the sheet is second roughness smoother than the first roughness. Further, as described above, the second parameter indicates the interval of unevenness with respect to the horizontal direction in the contour curve of the surface of the sheet. That is, the surface roughness of the sheet becomes rough with a smaller second parameter. That is, when the second parameter is a third value, the surface roughness of the sheet is first roughness, and when the second parameter is a fourth value smaller than the third value, the surface roughness of the sheet is second roughness smoother than the first roughness. In such a case, when the surface roughness of the sheet is the first roughness, the controller 301 sets the target pressure Pc of the pick-up roller 6 to the sheet at first pressure. Further, when the surface roughness of the sheet is the second roughness, the controller 301 adjusts the target pressure Pc of the pick-up roller 6 to the sheet so as to be second pressure larger than the first pressure. This is because there is a tendency that the value of the coefficient A in the (formula 1) becomes larger with a larger first parameter and further there is a tendency that the value of the coefficient B in the (formula 1) becomes larger with a smaller second parameter.

Embodiment 2

Next, an embodiment 2 as a modified embodiment of the embodiment 1 will be described. In this embodiment, the rotational speed of the pick-up roller 6 is adjusted depending on the surface property of the sheet. FIG. 11 is a flowchart showing a flow of an operation for adjusting the rotational speed of the pick-up roller 6. The flowchart of FIG. 11 is principally executed by the controller 301. In the flowchart of FIG. 11, steps from S1 to S5 are the same as those in the flowchart of FIG. 6. This embodiment is different from the embodiment 1 only in that the rotational speed of the pick-up roller 6 is adjusted on the basis of the relation curve showing the relationship between the feeding resistance and the feeding efficiency between the pick-up roller 6 and the sheet. In the description for FIG. 11, the steps up to S5 are the same as those in FIG. 6, and therefore will be omitted from redundant description.

As the case where a sheet feeding distance per unit rotation amount, i.e., the feeding efficiency, of the pick-up roller 6 is improved, the case where the rotational speed of the pick-up roller 6 is increased and the case where the rotational speed of the pick-up roller 6 is decreased exist. This changes depending on a combination of the difference (first parameter) between the highest point and the lowest point with respect to the height direction in the contour curve of the surface of the sheet with the interval (second parameter) of the unevenness with respect to the horizontal direction in the contour curve of the surface of the sheet. Part (a) of FIG. 12 is an example of a graph showing a relationship between the rotational speed of the pick-up roller 6 calculated from the (formula 1), the contact pressure of the pick-up roller 6 to the sheet, and the feeding state. Further, part (b) of FIG. 12 is another example of a graph showing a relationship between the rotational speed of the pick-up roller 6 calculated from the (formula 1), the contact pressure of the pick-up roller 6 to the sheet, and the feeding state.

A target value of the rotational speed of the pick-up roller 6 is set at a value (target speed Vc) obtained by adding a speed of a margin with a variation in sheet to a speed at the boundary, defining the stable feeding region and the unstable region, indicated by a solid line in each of parts (a) and (b) of FIG. 12 (S23). Here, the speed at the boundary defining the stable feeding region and the unstable region in the embodiment 2 corresponds to a rotational speed (boundary speed) of the pick-up roller 6 at which a sheet feeding distance per unit rotation amount of the pick-up roller 6 is a distance of a degree such that the non-feeding of the sheet does not occur. In part (a) of FIG. 12, a relationship of a feeding state with respect to the sheet having a surface property such that the feeding region is the stable feeding region when the rotational speed of the pick-up roller 6 is made slower than the boundary speed is shown. In S23, as regards such a sheet, the rotational speed of the pick-up roller 6 is set so as to be slower than the boundary speed. On the other hand, in part (b) of FIG. 12, a relationship of a feeding state with respect to the sheet having a surface property such that the feeding region is the stable feeding region when the rotational speed of the pick-up roller 6 is made faster than the boundary speed is shown. In S23, as regards such a sheet, the rotational speed of the pick-up roller 6 is set so as to be faster than the boundary speed. Further, a driving sequence of the feeding driving motor M3 for driving the pick-up roller 6 is changed so that the rotational speed of the pick-up roller 6 becomes the set target speed Vc (S24), and then a sheet feeding job is started (S25).

Incidentally, when the sheet is picked up by the pick-up roller 6, a feeding resistance acting between the sheets stacked on the sheet cassette 2 becomes larger with larger (rougher) surface roughness of the sheet. That is, with the larger surface roughness of the sheet, the sheet is readily picked up, and on the other hand, with smaller (smoother) rotational speed of the sheet, the sheet slips and is not readily picked up. In such a case, when the surface roughness of the sheet is the first roughness, the controller 301 sets the rotational speed of the pick-up roller 6 to the sheet at first speed. Further, when the surface roughness of the sheet is the second roughness, the controller 301 adjusts the rotational speed of the pick-up roller 6 to the sheet so as to be second speed higher than the first speed.

Incidentally, the constitution of the embodiment 2 can also be applied to a sheet feeding device provided with no mechanism for adjusting the contact pressure of the pick-up roller 6, such as the adjusting unit 14.

Other Embodiments

In the embodiments 1 and 2, the pick-up roller 6 was described as an example, but the pieces of control of the embodiments 1 and 2 are also applicable to other feeding rollers of the printer 201. Further, as the image forming means of the printer 201, it is possible to use a method, other than the electrophotographic type, such as an ink jet type.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2020-119986 filed on Jul. 13, 2020, which is hereby incorporated by reference herein in its entirety. 

What is claimed is:
 1. A sheet feeding device comprising: a sheet accommodating portion configured to accommodate a sheet; a feeding roller configured to feed the sheet by rotation thereof in a state in which said feeding roller contacts an uppermost sheet accommodated in said sheet accommodating portion; detecting means configured to detect a surface property of the sheet accommodated in said sheet accommodating portion; changing means configured to change contact pressure of said feeding roller to the uppermost sheet; and a controller configured to control said changing means so as to change the contact pressure on the basis of a result of the surface property of the sheet detected by said detecting means.
 2. A sheet feeding device according to claim 1, wherein the sheet has surface roughness including first roughness and second roughness larger than the first roughness, and wherein after said changing means changes the contact pressure, the contact pressure of said feeding roller when the surface roughness is the second roughness is contact pressure larger than the contact pressure of said feeding roller when the surface roughness is the first roughness.
 3. A sheet feeding device according to claim 1, further comprising urging means configured to urge said feeding roller toward the uppermost sheet accommodated in said sheet accommodating portion, wherein said changing means changes a force for urging said feeding roller by said urging means.
 4. A sheet feeding device according to claim 1, wherein said controller calculates feeding efficiency which is a sheet feeding distance per unit rotation amount by a detection result of the surface property of the sheet and changes the contact pressure by said changing means so that the feeding efficiency is a predetermined value or more.
 5. A sheet feeding device according to claim 1, further comprising separating means including a separation nip in which sheets fed by said feeding roller are separated one by one, wherein a feeding force generated by the contact pressure of said feeding roller is smaller than a sheet separating force generated in the separation nip.
 6. A sheet feeding device according to claim 1, wherein surface roughness of the sheet includes a first parameter indicating a difference between a highest point and a lowest point with respect to a height direction in a contour curve of a surface of the sheet, and is first roughness when the first parameter is a first value and is second roughness when the first parameter is a second value smaller than the first value.
 7. A sheet feeding device according to claim 6, wherein the surface roughness includes a second parameter indicating an interval of unevenness with respect to a horizontal direction in the contour curve of the surface of the sheet and is the first roughness when the second parameter is a third value and is the second roughness when the second parameter is a fourth value smaller than the third value.
 8. A sheet feeding device according to claim 1, wherein said controller is capable of controlling a rotational speed of said feeding roller, and wherein the rotational speed is a first speed when surface roughness of the sheet is first roughness and is a second speed higher than the first speed when the surface roughness is second roughness.
 9. A sheet feeding device according to claim 1, wherein said controller controls said changing means so that pressure obtained by adding pressure of a margin with variation in surface property on a surface of the sheet to pressure of a threshold at which a sheet feeding distance per unit rotation amount of said feeding roller is a predetermined distance is the contact pressure of said feeding roller.
 10. A sheet feeding device according to claim 1, wherein said detecting means includes a confocal laser optical element configured to laser-scan a surface of the sheet and a light receiving element configured to receive reflected light from the surface of the sheet laser-scanned by said confocal laser optical element, and outputs a signal depending on the surface property.
 11. A sheet feeding device according to claim 1, wherein said detecting means is disposed inside said sheet accommodating portion.
 12. A sheet feeding device according to claim 1, wherein said changing means includes a spring for generating the contact pressure by pressing said feeding roller, a stepping motor, and a moving member for expanding and contracting said spring by drive of said stepping motor.
 13. An image forming apparatus comprising: a sheet feeding device according to claim 1; and image forming means configured to form an image on the sheet fed by said sheet feeding device. 